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Tankless or boiler w/ indirect for DHW, radiant loop run off coil??

rbphhc
rbphhc Member Posts: 126
Hey all, question about heating/ hot water revamp:

Existing system has 2 forty-gal propane HWHs for domestic and radiant, respectively.

The radiant is a single loop, in-floor, one large rm @ 20x30.

Hydronic HWH failed, so we want to re-do whole system as tankless, and free up floor area.

Considering three options:

!/ Combi boiler to run both systems. Might be overkill, with only one heating zone? Plus, lotta money. Or:

2/ Boiler or tankless HWH w/ Taco X-block to run the single-zone hydronic;

3/ Tankless HWH or small wall-mounted boiler with indirect; HWH would maintain 41-gal indirect @ above DHW temp. DHW drawn from the 41 gal storage. Indirect *coil* to heat in-floor hydronic. DHW would be tempered down using mixing valve.

If that's not clear, pls see attached sketch (two systems separated for clarity, but only one boiler/indirect, of course).

Questions:

- Is it realistic that a tankless HWH could keep up with both DHW *and* the losses from in-floor heating?

- Is there any mechanical difference between a big tankless HWH and a small wall-mounted boiler? Are BTUs just BTUs? Aren't they both basically heat exchangers over burners? Flame + tubing = hot water. No?

- Are any elbows allowed in the exhaust vent, or is it just up and out?

- 2 circ pumps - one to run the indirect tank through the tankless or boiler, set to maintain a temp range in the tank, and one to circulate the radiant floor loop through the indirect coil. RIght?

- The big Q: how many BTUs, ballpark, needed to maintain indirect tank temps at @ 120f, even with floor water circulating through the coil, and maybe a couple faucets open? Three people in the home, 2 baths, VT climate, 70s home, not particularly tight.

Any other thoughts/ suggestions welcome.



Have at it!
«1

Comments

  • Hot_water_fan
    Hot_water_fan Member Posts: 1,802
    1. Best to save floor space. 

    For 3 - how about a 80kbtu wall mounted boiler, indirect and the central heating loop heats the slab? That allows condensing, which is the whole point of the condensing boiler. Like this picture, but 1 zone. 



    Is there any mechanical difference between a big tankless HWH and a small wall-mounted boiler? Are BTUs just BTUs? Aren't they both basically heat exchangers over burners? Flame + tubing = hot water. No?
    A tankless HWH is generally much larger than a boiler because it needs to make hot water on demand. That often exceeds the heat loss, especially for spaces 600 sqft. They’re different for other reasons too - get a combi if you want to go that route. 

    You need to figure out your heat loss. It’s probably smaller than any non-condensing boiler if you could meet the load with a typical hot water heater, but there’s some mass to keep the cycles long enough. How much propane did you use last year?
    rbphhc
  • hot_rod
    hot_rod Member Posts: 21,898
    One single 600' foot loop in that 20 x 30 room? So basically a 1/2 to 3/4 gpm load for the heat?
    Then your DHW is the larger and the load to size to. Sounds like a 35,000 water heater has cover the load so far.
    You have a unique diverse load, a tiny hydronic load, a small DHW requirement.

    Might look a one of the HTP tank products for both loads, something that has some buffer ability.

    Tankless doing both loads would not be on my option list.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    rbphhcGGross
  • Mad Dog_2
    Mad Dog_2 Member Posts: 6,768
    Bradford White Combi-Cor dual water heater and Radiant heating source .  Works well. Won't take up any New Space.  Just call Tech support and make sure the btu output and recovery will meet the radiant floor load...should.  I've done my that size with a 50 or 75 gallon model. Just an option.  Mad Dog
    rbphhcGGross
  • rbphhc
    rbphhc Member Posts: 126
    hot_rod said:

    Tankless doing both loads would not be on my option list.

    How about tankless doing both load WITH an indirect, as described above? If I keep the delta T large for the indirect temps (maybe maintain between 115 and 125?), wouldn't that protect the tankless from short-cycling too much? It would start a burn at 115 and not stop til the entire 41 gals was up to 125, then rest?
  • rbphhc
    rbphhc Member Posts: 126
    hot_rod said:


    Might look a one of the HTP tank products for both loads, something that has some buffer ability.

    Thanks! Interesting option, didn't know they had tank water heaters with coils. Do you know if they are double-wall? Think that's required for hydronic-DHW shared tanks in VT.
  • rbphhc
    rbphhc Member Posts: 126
    Mad Dog_2 said:

    Bradford White Combi-Cor dual water heater and Radiant heating source .  Works well. Won't take up any New Space.

    Good. SImilar to the HTP suggesting above? Appreciated.
  • rbphhc
    rbphhc Member Posts: 126
    Pretty much exactly this.
  • rbphhc
    rbphhc Member Posts: 126

    For 3 - how about a 80kbtu wall mounted boiler, indirect and the central heating loop heats the slab? That allows condensing, which is the whole point of the condensing boiler. Like this picture, but 1 zone.

    Thanks, HotWaterFan. I've been studying your schematic, and I don't quite understand the circulation pattern coming out of the boiler.

    This much I get:
    - domestic hot comes out of the indirect (I assume the tank, not the coil), and cold make-up water also goes straight into the indirect, and supplies the tempering valve;
    - hydronic loop is on its own circulator, with zones off that loop, opened by valves; clear enough;
    - And it looks like both the indirect (DHW) and the hydronic system have separate loops running through the boiler, with separate circulators; I assume DHW is running through the indirect coil; and looks like hydronic loop runs through a (low-loss) header?

    Beyond that, I'm a little confused, partially because the diagram only shows return to the boiler, not send (or shows them coming out of the same port).

    Obviously, not a plumber. Just a builder of 35 years and a homeowner. Would appreciate any clarification (or links) you can provide that explain *this* part of the sytem (image). Thanks!

  • rbphhc
    rbphhc Member Posts: 126
    edited February 2023
    We had two local plumbing/heating outfits come in yesterday to assess and make estimates. Both recommended a combi boiler for the upgrade.

    One came in at $XXk, the other at $XXXk. The landlord has a budget of $Xk. (EDIT: Advised to delete quote prices. Sorry! Basically, budget is 1/2 to 1/3 or quotes.)

    I am the property manager (35 year builder w/ some plumbing/heating experience). I have been running a Takagi Jr as a boiler for the hydronic system in my own home for years, w/ no issues. Works great.

    Question #1: is there any reason a tankless HWH could not maintain a 40gal indirect tank at appropriate temps for DHW?

    Question #2: Is there any reason a single hydronic loop couldn't also run through the coil in the indirect?

    This is the basic setup I want to recommend, if the LL can't afford a combi boiler. One of the techs yesterday said it would work great. It's just not something they would install (they make their money hawking Viessmann boilers) (which they priced at over $10k yestarday, btw).
  • GGross
    GGross Member Posts: 977
    @rbphhc

    You are not supposed to post the prices you are quoted on this forum, you can just X them out

    That being said if your installer has faith in a system, and the price is right for your budget, then go ahead and have them install it. The primary concerns most have with tankless units for space heating is that at it's core it is a misapplication. The equipment is designed to run at a very wide Delta-T and space heating likes around a 20 degree Delta T, infloor will use an even tighter delta.

    The other potential issue is that (and this is specific to the Takagi, which also rebrands for many other manufacturers such as AO smith/American) the tankless water heater is REQUIRED to be hooked up to domestic water, for tankless use. Meaning that using the Tankless heater to heat an indirect coil and do space heating, would be voluntarily giving up the warranty on the equipment should the question ever arise with tech support or the manufacturers warranty department.

    btw the price you posted for a small Viessmann with indirect heater is a steal. I would just jump on that one personally but I am a Viessmann wholesaler so a bit biased
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,802
    edited February 2023


    Looks like this - the source is the Lochinvar knight installation manual. You could locate a squat indirect directly underneath the boiler to save space.

    - domestic hot comes out of the indirect (I assume the tank, not the coil), and cold make-up water also goes straight into the indirect, and supplies the tempering valve;
    - hydronic loop is on its own circulator, with zones off that loop, opened by valves; clear enough;
    - And it looks like both the indirect (DHW) and the hydronic system have separate loops running through the boiler, with separate circulators; I assume DHW is running through the indirect coil; and looks like hydronic loop runs through a (low-loss) header?


    The indirect and central heating use the same boiler piping - it switches to DHW when that load calls. It's primary/secondary, but could be a low-loss header.

    Question #1: is there any reason a tankless HWH could not maintain a 40gal indirect tank at appropriate temps for DHW?

    Question #2: Is there any reason a single hydronic loop couldn't also run through the coil in the indirect?


    It's basically the same thing as the diagram above, just more complicated. Just use that if you want storage + condensing. I think the best solution is probably a storage tank rated for DHW and Central heating - like the Phoenix by HTP.

    Another option is to rethink the entire scheme - maybe electric baseboard will be the solution. More expensive to operate, but cheap upfront. With a low heat loss, the energy cost becomes a smaller concern compared to capital cost.
    GGross
  • rbphhc
    rbphhc Member Posts: 126
    GGross said:

    You are not supposed to post the prices you are quoted on this forum, you can just X them out

    The primary concerns most have with tankless units for space heating is that at it's core it is a misapplication. The equipment is designed to run at a very wide Delta-T and space heating likes around a 20 degree Delta T, infloor will use an even tighter delta.

    Sorry about the quotes! Fixed.

    RE: Delta T - Thanks for the explanation.
    I was thinking you could configure the indirect to maintain a large delta - say from 110 to 150 - and used cold to temper the DHW, and return to temper the in-floor.
    But then you'd potentially have lukewarm HDW, and too-hot floor.
    And a tighter Delta (say 120 to 130) is even less optimal for the tankless and still too hot for the floor.
    So clearly makes more sense to run a boiler.

    I like the squat indirect under an up-high boiler. Thx!

  • hot_rod
    hot_rod Member Posts: 21,898
    Rinnai actually builds a few tankless mounted on tanks, an 80 and 120 tank option 199K boiler. I saw a few yesterday at the homebuilders show. High demand, large dump loads like all the carwash type shower heads on display around the show floor!
    So that part is workable.

    Depending on the load of the heating from that upper coil in the tank. The coil is sitting in stagnant water, so once the coil temperature drops, you don't get much heat transfer. And with the control at the bottom, the entire tank needs to cool the 15° differential to kick the heater back on. If the circ on the tankless runs constantly, that would help heat exchange a bit.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    rbphhc
  • rbphhc
    rbphhc Member Posts: 126
    It occurs to me that we could use the secondary (hydronic in-floor) loop as a pre-heater for the DHW, both to cool the floor water, and also to ease loads on the boiler.

    So the boiler could be maintaining an indirect at 120°f, say, and the in-floor loop running through the *coil* in the indirect could also run through a tempering valve before entering the floor, transferring some of its heat to the CWS to the boiler, and maintaining floor temps at @ 90°f.

    OR, I wonder if we could run JUST a simple tankless HWH w/ no indirect? I.e. Cold water > tempering valve to cool *floor* water and preheat tankless input > gets heated to 130*f or so > runs through FPHE to transfer some heat to hydronic floor loop. Ideally we want 120 to DHW, 90 to floor - is it possible to control both digitally? Modulate the tankless? Would a Taco X-block do the trick?

    That way, tankless HWH would be operating normally at high Delta T - in cold, out hot, periodically. But we would still need some kind of storage, b/c floor needs 90f continually, DHW needs are sporadic. So maybe a small buffer tank?

    Thoughts?
  • rbphhc
    rbphhc Member Posts: 126

    Another option is to rethink the entire scheme - maybe electric baseboard will be the solution. More expensive to operate, but cheap upfront. With a low heat loss, the energy cost becomes a smaller concern compared to capital cost.

    Thanks! Landlord will never go for that. She is very proud of in-floor hydronics. I need a simple, space-efficient way to provide modest on-demand DHW + small hydronic load to one 600sf space. Burning up pencil erasers here.
  • hot_rod
    hot_rod Member Posts: 21,898
    it’s a we have all been looking for a small load boiler/ heat source
     For those jobs with under a 20K or smaller design day load 

    when you add efficient, small, DHW, etc etc to the wish list, the “affordable” parts starts slipping away

    Which is what drives basic plain gas or electric DHW water heater tanks to be used for small radiant load type of systems. It checks all the boxes except small and efficient
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    rbphhc
  • rbphhc
    rbphhc Member Posts: 126
    DHW is always running, albeit intermittently.

    How do I scavenge a *small* amount of heat from DHW, continuously, to meet 90*f hydronic in-floor needs, and keep stable floor temps, *while* providing that intermittent 120°f, on demand, to any faucet in the house?

    Would a small well-insulated buffer tank that is *recharged* automatically any time any DHW is used be enough to allow hydronic to draw from, without cooling too much and falling below 120*f?

    (I.e. DHW buffer would not circulate constantly through the tankless - would just count on DHW use to recharge buffer. Obviously, if DHW is not used, like people are out of town for days, buffer would slowly fall to ambient temps - but that would be fine to keep anything from freezing, no?)

    Another thing to consider: rooftop PV system going in this spring, so electricity is not a bad backup option, either heat pump or tankless. I.e. small tankless could be used to make sure 120*f at point-of-use, or small HWT to maintain 90*f in-floor and/or function as buffer(?)

    Here is a sketch of current thinking:
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,802
    edited February 2023
    If you’re going with a wall mounted boiler, just use the lochnivar set up or a combi. Don’t reinvent the wheel. The landlord might have to increase the budget. 

    Or maybe an electric boiler and a separate electric tank for DHW. 
    rbphhckcoppDerheatmeisterGGross
  • rbphhc
    rbphhc Member Posts: 126
    hot_rod said:

    Which is what drives basic plain gas or electric DHW water heater tanks to be used for small radiant load type of systems. It checks all the boxes except small and efficient

    Exactly.
    But we do have a small, efficient heat source in the form of a tankless HWH. Just needs to run at a high Delta.
    So I think treating DHW as the main load, and the hydronic as a 'piggy-back' secondary load makes sense.
    The thing we need is a small buffer tank with integrated FPHE or coil, or some other way hydronic can scavenge the heat it needs w/o over-cooling the primary DHW.
  • rbphhc
    rbphhc Member Posts: 126

    If you’re going with a wall mounted boiler, just use the lochnivar set up or a combi. Don’t reinvent the wheel. The landlord might have to increase the budget. 


    Or maybe an electric boiler and a separate electric tank for DHW. 

    I appreciate that. And it may be possible to install a combi (or the Lochnivar setup) w/o busting the budget by leveraging incentives, or good used equipment (wild west up here in VT).
    As for reinventing the wheel, I may have no choice. But that's how wheels get better!
  • rbphhc
    rbphhc Member Posts: 126
    Anyone know of a small buffer tank that can transfer heat?
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,802
    I don’t see what your plan provides as an improvement besides using a DHW appliance for a CH application. 
    rbphhc
  • rbphhc
    rbphhc Member Posts: 126

    I don’t see what your plan provides as an improvement besides using a DHW appliance for a CH application. 

    Well, maybe it provides a model for scavenging a small amount of heat for low-temp hydronic from high-load DHW.

    I'm a big fan of not wasting energy, and if most of the energy is going into DHW, why not steal a little for secondary use? Basically like taking an extra shower. No harm no foul.

    See revised concept below...
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,802
    You’re not scavenging energy. If you didn’t have the in floor heating, you’d need less energy. 
    rbphhc
  • rbphhc
    rbphhc Member Posts: 126
    edited February 2023
    Latest concept: stick a FPHE between the output of the TANKLESS and the output of the INDIRECT. That way DHW is always at 120* w/o tempering or electricity.
    AND there's a buffer tank for the hydronic to scavenge off of that never gets above 120* or below ambient. Getting simpler and cheaper.
    Temper valve between hydronic send and return, so floor can circulate w/ little or no additional heat if it gets too hot.
    Optional temper valve for DHW if temp needed above 120°F (not sure why you would).
    In summer, hydronic is dormant, and indirect becomes buffer for DHW. Additional capacity.
    In winter, hydronic is active and tankless has to work a little harder. But still 70* Delta T. Should be happy.
    Question: why is doing hydronic in a side-arm application like this a bad idea? Why does it void warrantees? Tankless HWH is being use exactly as designed - making DHW from CWS. Hydronic load no different than pouring a tub and letting it cool.

  • Hot_water_fan
    Hot_water_fan Member Posts: 1,802
    A combi already has a heat exchanger, as does an indirect. You’re asking an installer to build a combi in the field, which will not be cheaper and adds a redundant (efficiency sapping) second heat exchanger. 

    The concrete will act as a buffer, so you don’t need to buffer the central heating load. Now decide if you want a buffer for DHW (an indirect) or a just-in-time DHW (a combi). 
    rbphhc
  • rbphhc
    rbphhc Member Posts: 126
    edited February 2023

    You’re not scavenging energy. If you didn’t have the in floor heating, you’d need less energy. 

    Totally true. By 'scavenging' I just meant grabbing a little off the DHW, since the hydronic load is so small. But yes, the HWH would definitely burn a little longer to make up those BTUs. No free lunch.
  • rbphhc
    rbphhc Member Posts: 126

    A combi already has a heat exchanger, as does an indirect. You’re asking an installer to build a combi in the field, which will not be cheaper and adds a redundant (efficiency sapping) second heat exchanger. 


    The concrete will act as a buffer, so you don’t need to buffer the central heating load. Now decide if you want a buffer for DHW (an indirect) or a just-in-time DHW (a combi). 
    True, on both counts. Floor definitely a buffer.
    And combi was first choice, been pushing that for weeks.
    Problem is, LL budget precludes a new high-quality combi. Estimates we are getting are 2x and 3x her budget.
    So yes, basically trying to build a small, efficient combi in the field. Re-using parts to stay within budget - existing indirect, expansion tank, circ pump, etc. She can afford a new tankless HWH.
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,802
    Question: why is doing hydronic in a side-arm application like this a bad idea? Why does it void warrantees? Tankless HWH is being use exactly as designed - making DHW from CWS. Hydronic load no different than pouring a tub and letting it cool.
    How does the system heat the slab when there’s no DHW demand? 
    rbphhc
  • hot_rod
    hot_rod Member Posts: 21,898
    You would need to run a recirculating pump to keep the tank hot, of course. Maybe put a plate HX in that recirc loop, it stays at the same temperature as the tank. Then pull the radiant off the other side of the HX.
    I don’t know how accurately the tank temperature will be maintained.
    Keep the tank and tankless at 140, pull the radiant off the plate HX via a thermostatic mix valve.
    The recirc pump needs to be sized according to the pressure drop of the tankless. So it will require a stainless pump, perhaps a high head model.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    rbphhc
  • rbphhc
    rbphhc Member Posts: 126
    edited February 2023

    How does the system heat the slab when there’s no DHW demand? 

    It doesn't. Well, it does for a while. Tentative thinking:

    1/ When people are home, they use hot water. Dishes, laundry, showers, etc. It's pretty constant use, all day long. So I'm thinking that 'sidearm' indirect will stay pretty well charged with at least warm water.

    2/ At night, no hot water use. But people usually turn down heat at night. And the slab holds heat for a good 24 hrs, so I'm not too worried about it 'coasting' through 6 or 8 hrs. It'll ramp right back up with AM showers.

    3/ Also, I can run the tankless at 130 or 140f, if needed, to give that indirect more of a 'charge' if intermittent DHW use isn't doing the trick. Will have to fine-tune, of course, once it's up and running.

    4/ The DHW has the FPHE to make sure it stays up to temp. And tempering is possible for both DHW and hydronic. I like the idea of the 'passive' indirect. I'm curious if it will work. I have a surplus Takagi and an extra Amtrol sitting around. If it doesn't actually function, back to the boiler + indirect or combi.

    Any other fatal flaws with the system anyone sees? Thanks!

  • rbphhc
    rbphhc Member Posts: 126
    hot_rod said:

    You would need to run a recirculating pump to keep the tank hot, of course. Maybe put a plate HX in that recirc loop, it stays at the same temperature as the tank. Then pull the radiant off the other side of the HX.
    I don’t know how accurately the tank temperature will be maintained.
    Keep the tank and tankless at 140, pull the radiant off the plate HX via a thermostatic mix valve.
    The recirc pump needs to be sized according to the pressure drop of the tankless. So it will require a stainless pump, perhaps a high head model.


    Digesting. Stand by. Thanks!
  • rbphhc
    rbphhc Member Posts: 126
    hot_rod said:

    You would need to run a recirculating pump to keep the tank hot, of course. Maybe put a plate HX in that recirc loop, it stays at the same temperature as the tank. Then pull the radiant off the other side of the HX.
    I don’t know how accurately the tank temperature will be maintained.
    Keep the tank and tankless at 140, pull the radiant off the plate HX via a thermostatic mix valve.
    The recirc pump needs to be sized according to the pressure drop of the tankless. So it will require a stainless pump, perhaps a high head model.

    But recirculating the tank through the tankless would mean short-cycling it, right? That's what I was trying to avoid with the 'passive-indirect' design.

    I'm thinking the buffer of the tank PLUS the buffer of the mass in the floor might be enough to smooth over DHW fluctuations.

    I.e. not necessary to circulate tank through tankless. Just count on enough DHW use to keep tank warm, ish. We only need 90* for floors, right?

    And FYI - we haven't had heat in the floor for over a month, through -20f weather up here in VT, and the room is still fine. Floor doesn't even feel that cold. It's well-insulated below, sitting on the earth, slab on grade. So there may be enough 50* geo temps, at least toward the middle, to keep that floor from getting icy.

    There's also a mini-split in the room (doesn't work too well in sub-zero temps, but it's trying). All that to say, we don't need perfect hydronic temps. If they dip (or spike) the mass will smooth 'em out. And anything would be better than nothing (what we have now).
  • rbphhc
    rbphhc Member Posts: 126
    I'm noticing that Rinnai offers tankless HWH w/ built-in 'Smart Circ' recirculation as a feature. I'm wondering if it would work to install their 'thermal bypass valve' instead of the FPHE bridging the output of the tankless and output of the indirect. That way, periodic recirculation would 'refresh' the hot charge in the indirect tank (w/ hydronic coil). On the other hand, reviews say this is a cheap plastic part that fails regularly. So maybe stick to original design. Thoughts?
  • rbphhc
    rbphhc Member Posts: 126
    Two options, as of today. One re-circulates the indirect, one doesn't.

    Recirculating indirect would act like just another DHW load, like filling up a bathtub. Every so often, the tank would reach down to 90* and call for heat. The pump would kick on, and circulate the indirect loop until 130* was reached.

    (Only difference is, incoming H2O would be at 90* instead of 50*. How bad is that if outgoing is 130* ish?)

    That circ pump could be set for slow speed so it didn't interfere much with any DHW draw that happened to come at the same time. Or it could be timed to recirculate at night, when no DHW use.

    Recirculating keeps the indirect more independent of the DHW in two ways: 1/ temperature independent, obviously; but also 2/ more water independent, since DHW is not being forced through the indirect. Rather, it functions somewhat like a closed loop, since it only circulates during recharge.

    I.e. If no DHW load comes in during recirc, there's not much mixing. Even though fittings are open, DHW would stay in its own 'path,' and hydronic in its own. They only share passages through the HWH.

    Benefit of passive indirect (snake-ate-the-hippo) is: it's simpler. No extra circ pump or hydronic loop. Also keeps Delta T very high for the tankless, since that's what it prefers (correct?)


  • rbphhc
    rbphhc Member Posts: 126
    Any further thoughts? LL is balking at the pricetag for combi boiler. She's back to wanting 2 fresh 40 gal propane HWTs, which was the original system that failed. No pro will install an old-style tank for heat, and likely not for DHW either. Money is the sticking point.
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,802
    The issue doesn’t seem to be technical - we’ve provided many functional, reliable options. Radiant floors are not new - the landlord needs to follow a realistic (and legal path) or sell the property to someone who will. 
    Canuckerrbphhc
  • rbphhc
    rbphhc Member Posts: 126

    The issue doesn’t seem to be technical - we’ve provided many functional, reliable options. Radiant floors are not new - the landlord needs to follow a realistic (and legal path) or sell the property to someone who will. 

    Agreed! And thanks very much for all the excellent advice!

    But it's not illegal, or unrealistic, AFAIK, to use a hot water heater of some kind for radiant heat, is it? Pretty sure not in rural VT, anyway, where no inspection is required, unless hooked up to town gas or water.

    And certainly it works - they got 31 years of trouble-free service out of the previous system.

    Even in the real world, it might void the warrantee, or burn out from short-cycling, but I don't think it's literally against the law??
  • rbphhc
    rbphhc Member Posts: 126
    BTW, a discussion here from @15 years ago suggested similar solutions to what I posted above: https://forum.heatinghelp.com/discussion/105764/radiant-with-bradford-white-combi-core-water-heater
  • rbphhc
    rbphhc Member Posts: 126
    edited February 2023
    For anyone following the thread, here's one guy's take. Heat + DHW for $20/mo, using a Rinnai tankless as only heat source. (FTR, I have questions about this design, as it circulates a 'primary loop' through the tankless anytime there is a call for heat or DHW. Seems like a recipe for short-cycling. Makes more sense to me to send DHW directly through the HWH and run a single loop for radiant only. But I could be wrong. Maybe his primary cools down significantly between burns, and runs only occasionally at a high Delta.) https://www.jlconline.com/how-to/plumbing/heating-a-home-with-a-tankless-water-heater_o